The present invention relates generally to the field of disc-drive data storage systems and, in particular, to track search capability within such systems.
The term "disc-drive systems" (or "disk-drive systems") is directed to any system (e.g., optical, magnetic, etc.) that accesses data held on a rotating disc. Optical disc-drive systems include read-only compact discs (CD), digital versatile discs (DVD) and digital videodiscs (DVD), as well as their writable counterparts (e.g., CD-R, CD-RW, DVD-R and DVD-RAM). In such systems, information is read from and/or written to a disc by a transducing head or "pickup" supported adjacent the disc surface. Among the most common of these optical systems is CD-ROM.
CD-ROMs store data in a single, spiral track (analogous to a phonograph record) that circumnavigates the disc thousands of times (e.g., over 20,000) as it gradually moves away from the center of the disc. For ease of discussion, each rotation of this single, spiral track is referred to herein as a track. The architecture and operation of CD-ROM drive systems are well-known by those having ordinary skill in the art, and a description of such systems may be found in C. Sherman, CD-ROM Handbook, Intertext Publications McGraw-Hill, Inc. (1994), which is hereby incorporated by reference in its entirety for all purposes.
In conventional CD-ROM drive systems, CD-ROMs are "read" with a laser beam emitted from an optical pickup suspended beneath the disc. The disc reflects the emitted beam back towards the pickup which contains photodiodes to detect the intensity of the reflected beam (modified by surface irregularities on the disc). The reflected beam conveys both data and tracking information.
A conventional CD-ROM drive system typically includes a positioning servo loop or, more generally, a servo system (containing one or more servo loops) to position and maintain the pickup precisely over a selected track of the disc. The operation of maintaining the pickup over a desired track is referred to as "track following". The operation of positioning the pickup over a destination or target track by laterally moving the pickup across one or more tracks is referred to as a track "search" or "seek."
In a track search operation, pickup position relative to the disc is determined by monitoring the number of tracks crossed over (i.e., "track crossings") as the pickup moves from a starting track to a destination track. This monitoring is performed by the photodiodes of the pickup.
FIG. 1 illustrates an exemplary layout of photodiodes within an optical pickup 100. As shown in this figure, four photodiodes 102-108 (which generate signals A, B, C and D, respectively) are clustered together at the center and two photodiodes 110, 112 (which generate signals E and F, respectively) are staggered diagonally on the periphery. During a track search, this array of six photodiodes moves in a direction shown by arrow 114, which is perpendicular to track orientation arrow 116 (approximating track orientation on an optical disc).
The diode arrangement of FIG. 1 is configured to facilitate a "three-beam" laser operation, which is well known to those having ordinary skill in the art. In such operation, a single laser beam generated within the disc drive system passes through a diffraction grating plate to produce two small side beams (e.g., side beams 370 and 372 of FIG. 5) on either side of a single main beam (e.g., main beam 330 of FIG. 5). All three beams are then emitted from a pickup onto a rotating disc, which reflects the beams back to photodiodes disposed within the pickup. This three-beam operation is commonly used in CD-ROM drive systems as well as CD audio systems.
During conventional three-beam operation, the main beam is reflected off a rotating disc and detected by diodes 102-108. The signals generated by these diodes may be summed together (producing signal "A+B+C+D") through techniques well-known to those having ordinary skill in the art. The resulting signal oscillates in conjunction with track crossings during a track search.
Further, each periphery diode 110, 112 detects one of the side beams reflected off the rotating disc (each slightly off track). The signals generated by these diodes are subtracted from each other (producing signal "E-F") again through techniques well-known to those having ordinary skill in the art. The resulting signal also oscillates in conjunction with track crossings. Accordingly, during a track search, signals A+B+C+D and E-F function as track-crossing signals representing pickup movement across one or more tracks.
In a conventional track search, pickup 100 crosses tracks of an optical disc by moving along the radius of the disc in the direction of arrow 114. During this process, signals A+B+C+D and E-F ideally result in sine waves 90.degree. out of phase from each other, oscillating about a reference voltage "V.sub.ref " as shown by waveforms 202 and 204 in FIG. 2A. V.sub.ref is typically ground or an offset ground in a single power supply system.
The physical relationship of signal E-F with optical disc tracks is shown schematically in FIG. 2B. Referring to this figure, E-F 204 crosses a level zero 220 (i.e., V.sub.ref) at the center of tracks 210 and 212. As shown therein, the period "T" of E-F 204 represents the crossing of one track width or pitch (e.g., 1.6 .mu.m).
Ideally, signals 202 and 204 should have a 50-50 duty cycle, which contributes to more accurate detection of track crossings. Signals 202 and 204 of FIG. 2A are shown in this ideal state; i.e., they are above level zero 220 for about one half of their period and below this level for the other half (thereby representing a symmetric or 50-50 duty cycle).
However, in practice, this 50-50 duty cycle may not initially be achieved due to an unwanted DC bias on the subject analog signal (i.e., A+B+C+D and/or E-F) which creates an offset from symmetric operation. As is well known, this unwanted bias may be substantially nullified by applying a correction voltage or bias to the subject signal.
Referring to FIG. 3, a sinewave 402 (representing E-F in this example) is subject to an offset 406 from V.sub.ref. In accordance with conventional methods, this offset is determined by peak detecting the top and bottom of wave 402. Since signals from photodiodes and preamplifiers are noisy, average top and bottom peak values are calculated over a relatively large number of periods (i.e.,"T" of FIG. 4) of the subject wave. Typically, the peaks of thirty-two or sixty-four full sinewaves (generated over thirty-two or sixty-four periods, respectively) are measured to obtain the necessary values for calculating the offset. Once calculated, a correction bias 404 is adjusted to produce a new correction bias 404', which compensates for the undesired offset 406. The corrected wave 402'achieves an approximate 50-50 duty cycle about V.sub.ref.
The foregoing conventional method requires considerable time (i.e., 32 or 64 sinewave periods) to collect the required samples for averaging peak values. As such, this method has an inherent latency that is problematic when performing a track search operation since undesired offsets of A+B+C+D and/or E-F typically undergo rapid change during such searches.
Moreover, the foregoing conventional method is typically applied only once at spin-up calibration (i.e., during power up of a disc-drive system). However, optical pickup signals such as RFRP and TE have been observed to gradually deteriorate during the course of a track search when the pickup is in motion. As such, an initially-applied correction bias may be gradually rendered ineffective over the life of a single search.
Additionally, the conventional method is highly sensitive to sinewaves A+B+C+D and E-F being clipped or similarly distorted since an accurate offset can only be determined from accurate peak values.
Further, the conventional method produces a correction signal that is applied in its entirety at one time. If the offset is large, a comparable correction signal can introduce large transients into the servo loop used for tracking operations which may cause tracking reliability problems.
Thus, it would be desirable to correct for undesired offsets introduced into analog signals such as RFRP and TE in a manner that could respond dynamically to changes in the offset value during the course of a track search, be relatively insensitive to clipping or similar distortions of the analog signals and could introduce such correction gradually.